ATECC508A datasheet(83/109 Pages) MICROCHIP

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If reading a data zone and the EncryptRead bit is set in the corresponding SlotConfig word, the following

actions are taken to encrypt the data:

•

All of the TempKey register bits must be properly set as follows or else this command returns an

error:

TempKey.Valid == 1

TempKey.GenDigData == 1

TempKey.KeyID == SlotConfig.ReadKey

TempKey.SourceFlag == “Rand”

•

XOR the data from the memory zone with TempKey. Return as Contents.

9.17

SHA Command

Computes a SHA-256 or HMAC/SHA digest for general purpose use by the system. It may also be used

by the Verify(ValidateExternal) command to verify an X.509 certificate and store that validation

status with an internally stored public key.

Calculation of a SHA-256 digest occurs in the following three steps:

Start: Initialization of the SHA-256 calculation engine and initialization of the SHA context in

memory. This mode does not accept any message bytes.

Update: The command can be called a variable number of times with this mode to add bytes to the

message. Each iteration of this mode must include a message of 64 bytes. A variation on

SHA(Update) is SHA(Public) which inserts the contents of a public key slot into the message.

End: The SHA-256 calculation is completed, and the resulting digest is placed into the output

buffer. It is also stored in the TempKey register for subsequent (optional) use by the

Verify(ValidateExternal) command. From 0 bytes to 63 bytes may be passed to the device

for this mode.

On any error return code, or if any command other than SHA is sent to the device, the internal SHA

context is invalidated and TempKey is also invalidated.

This command can also optionally generate a digest to be used by Verify(ValidateExternal) to

validate a stored public key, which allows speed-up for future signature validations when X.509 format

signatures are used. To implement this, a SHA(Public) iteration can be inserted prior to SHA(End)

iteration, and the 64 bytes of the public key stored in the slot designated by Param2 will be added to the

message. This mode will fail if the designated slot does not contain a public key.

Verify(ValidateExternal) will only successfully validate the stored public key if the SHA(Public)

iteration occurs at the at the block number indicated by X509format<KeyConfig.X509id>.Bits<3:0> within

the sequence of SHA(Update) commands, and if the total number of blocks passed to the SHA

command matches the value in X509format<KeyConfig.X509id>.Bits<7:4> . This restriction prevents a

generation of non-standard X.509 templates, which may push the inserted public key into an unchecked

area of the template.

Calculation of an HMAC digest occurs in the following three steps, which are similar to the SHA process

above with the exception that a key slot is specified in the first step, and its value is used in the

calculations at the beginning and end of the message per the HMAC specification.

ATECC508A

Security Commands

Datasheet Complete

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